The Z-machine gets hot

Sandia National Labs reports some very unpleasant local temperatures, in …

The Z-machine at Sandia National Labs, pictured at right, was built in part to study the incredibly high pressures and temperatures involved in nuclear fusion. It achieves this through vaporizing a sample with a sudden jolt of energy, with the resulting plasma subject to intense magnetic pressure that compresses its ions down as close together as physics will allow. Normally, the sample used is a fine spool of tungsten wire, and the energy used is in the area of 20 million amps. But the scientists were not prepared for what happened when they changed over to a larger target composed of steel: temperatures rocketed above 2 billion degrees Kelvin, and the resulting release of X-rays pumped out more energy than the 20 million amps they put in. They had to repeat the experiments several times to make sure the results were real.

Why is this so surprising? Getting more energy out than what was put in would suggest a nuclear reaction of some sort was taking place. But fusion shouldn't be taking place under these conditions and besides, steel is mostly iron, and iron fusion actually draws in energy rather than releasing it. Left with no other explanation, the researchers propose that the sample is extracting energy from the magnetic field that contains it. They suggest that, instead of compressing evenly, the sample is subject to vortexes and internal friction that the magnetic field is forced to overcome. The energy input required to overcome that friction is what pushes temperatures so high. If samples of more fusable elements, such as deuterium, are subject to the same friction, then the energy input required to kick off fusion in such a system may be lower than expected.